Self Assessment Tax Return: A Small Business Owner’s Guide for 2026/27
If you’re running your own business in Haslingden or anywhere across Lancashire, self assessment is something you’ll deal with every single year. Yet it’s one of the tasks that seems to catch people off guard more than any other.
We file hundreds of self assessment tax returns each year at Nava Accountancy, and every January we hear the same thing: “I didn’t realise I needed to do that.” So we’ve put together this practical guide to help you stay ahead of the deadlines and avoid the most common pitfalls – whether you’re filing your 2025/26 return or preparing for the 2026/27 tax year ahead.
Who needs to file a self assessment tax return?
If you’re a sole trader, a partner in a business partnership, or a company director who receives income outside of PAYE, you’ll need to file. But it’s not just self-employed people. HMRC also expects a return from you if you:
- Earned more than £150,000 in the tax year
- Had untaxed income – rental properties, investment gains, or freelance work alongside your main business
- Need to claim certain tax reliefs
- Received child benefit and your income exceeded £60,000
The one that surprises people most? We regularly meet limited company directors who didn’t realise they needed to file a personal self assessment tax return alongside their company accounts. If you take dividends from your company, you almost certainly need to file one.
Key dates you need to know
For the 2025/26 tax year (which ended on 5 April 2026), here are the deadlines to mark in your calendar:
- 5 October 2026 – deadline to register for self assessment if this is your first return
- 31 October 2026 – deadline for paper returns (though very few people file on paper now)
- 31 January 2027 – deadline for online filing AND payment of any tax owed
- 31 July 2027 – second payment on account due, if applicable
The January deadline is the one that causes the most stress. Every year without fail, we see a rush of enquiries in the first two weeks of January from business owners who’ve left everything to the last minute. If you’re reading this now, you’ve got time on your side – and that’s a genuine advantage worth using.
What you’ll need to get your return right
Gathering your records before you sit down – or before you hand things over to your accountant – makes the whole process considerably smoother. Here’s what we typically ask our clients to prepare:
- Your UTR (Unique Taxpayer Reference) – the 10-digit number HMRC issued when you registered
- Records of all business income for the year
- Bank statements covering 6 April 2025 to 5 April 2026
- Receipts or records for allowable business expenses – we’ve written a separate guide on the ones people commonly miss
- Details of any other income: rental, savings interest, dividends, or freelance work
- Student loan details, if applicable
- Pension contribution records
- Any Gift Aid donations you’ve made
If you’re using cloud accounting software like Xero or FreeAgent, most of this will already be organised. That’s one of the reasons we encourage every business we work with to get their bookkeeping digital – it takes the scramble out of self assessment season entirely.
The mistakes we see most often
After filing thousands of returns over the years, certain patterns come up again and again:
Forgetting payments on account. If your tax bill exceeds £1,000 and less than 80% was collected at source, HMRC will ask you to make advance payments towards next year’s bill. This catches first-time filers off guard because they’re effectively paying around 150% of their expected bill in that first year – the current year’s tax plus half of next year’s estimate.
Missing the £1,000 trading allowance. If you have a small amount of self-employed income under £1,000, you may not need to declare it at all. But many people file unnecessarily – and others who should file don’t realise the threshold exists.
Not claiming all allowable expenses. This is the most common one by far. We had a client last year who’d been filing their own returns for three years and had never claimed mileage, use of home as office, or professional subscriptions. When we took over and reviewed those returns, the combined saving came to over £2,800. That’s money they’d simply left on the table.
Mixing up tax years. The UK tax year runs 6 April to 5 April – not January to December, and not April to March. Getting the dates wrong means your figures won’t reconcile with HMRC’s records, which can trigger enquiries you’d rather avoid.
What’s changing with Making Tax Digital
From April 2026, Making Tax Digital for Income Tax Self Assessment (MTD for ITSA) is being introduced for sole traders and landlords with income over £50,000. This means quarterly digital submissions to HMRC instead of a single annual return – a significant shift in how self assessment works.
If your income is between £30,000 and £50,000, you’ll be brought into MTD from April 2027. For those below £30,000, the timeline is still being confirmed, but the direction is clear. We’ve covered MTD in more detail in a previous post if you’d like to understand the full technical requirements.
Getting help with your self assessment
Filing your own return is entirely possible, but the reality is that most of the small business owners we work with find the process either stressful, time-consuming, or both. The tax system isn’t designed to be intuitive, and the penalties for errors or late filing add up quickly – £100 immediately if you miss the 31 January deadline, with daily penalties kicking in after three months.
At Nava Accountancy, we handle self assessment for businesses across Rossendale and Lancashire as part of our personal tax and wealth service. We don’t just file the numbers – we review your position, make sure you’re claiming everything you’re entitled to, and flag anything that could reduce your bill or improve your planning for the year ahead.
If you’d like to talk through your situation, get in touch for a free initial consultation and we’ll take it from there.
